1. Field
The present invention relates to an image encryption and decryption technique for preventing information leakage to a third party by visually encrypting a portion of image such as an important part or the like for a digital image and an image printed on printed matter.
2. Description of the Related Art
Aimed the progress of the information age, leakage of secret information has become a serious problem and hence the development of techniques to prevent information leakage is needed. For digital data, for example, techniques have been developed for encrypting data so that the content will not be visible if information is taken by a third party; some of these techniques are already utilized as useful unit for preventing information leakage.
Meanwhile, techniques for preventing the leakage of information from preprinted matter printed on paper and such have not been sufficiently developed, nor is there an example of a commercial product. Half of all information leakage is said to be related to printed matter, and therefore the development of a technique to prevent information leakage, as was done for digital data, is urgently required.
Examples in which countermeasures to information leakage from printed matter are required include bills issued at the time merchandise is purchased, credit card account statements, patient cards at hospitals, school report cards, and lists of names. The present invention is applicable to be used as a technique for preventing information leakage by encrypting an important part of those printed matters.
As a known example of encryption of printed matter in the conventional technique, there is a Patent Document 1, for example. In the Patent Document 1, first, an entirety of the image is divided into a plurality of blocks, and images of the divided blocks are rearranged based on a parameter obtained by an input password (encryption key). Further, the image of the block specified by the parameter is encrypted by black-and-white inversion and mirror inversion. In order to decrypt the encryption image, a frame for positioning is added outside of the image and the password (encryption key) is inputted. Then an original image is decrypted by a reverse procedure to that of the encryption.
Furthermore, such as a Patent Document 2, there is another conventional technique for imaging binary data to be embedded in printed matter. The imaging by this conventional technique can be achieved by presenting the binary data in a black-and-white square of a specified size and arranging the binary data in a matrix form. Moreover, in order to indicate a position in which the encryption is performed at the time of decryption, the printed matter is added with a symbol for positioning in a specified position of the matrix. Based on this symbol for positioning, it is possible to decrypt information embedded by taking an image with a scanner, a camera or the like.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 1996-179689.
[Patent Document 2] Japanese Patent No. 2938338.
However, the above described technique has the following problem to solve.
For example, in the technique such as the Patent Document 1, the encryption is applied only to the entirety of the image. The problem is that it is impossible to perform the encryption efficiently in a case when the region to be encrypted in the entirety of the image is very small.
Furthermore, in the technique such as the Patent Document 1, a frame for positioning outside of the encryption image needs to be added, which causes a problem that the image information which originally exists in the position where the frame is added is overwritten at the time of encryption.
Moreover, since the technique such as the Patent Document 1 does not consider distortion inside of the image, it is impossible to correctly detect blocks as the encrypted image becomes larger.
In the technique such as the Patent Document 2, the data containing little information such as text information can be embedded. However, this technique is not suitable for storing the data containing much information such as an image or audio information as well as having no difficulty if some decryption errors occur. Moreover, there was a problem that the data had to be a certain size and a square shape, and thus this technique was not applicable to an application which hides part of a character.
Based on an assumption that monochrome characters or diagrams are applied in the technique such as the Patent Document 2, there was a problem that this technique was not applicable to a color image such as a photo.